1. Field of the Invention
The present invention relates to electric actuators having an electric motor and an output shaft driven by the motor. More particularly, the present invention relates to an electric actuator which reciprocates an output shaft in a linear manner.
2. Description of the Related Art
Typically, automobile doors are provided with opening mechanisms. Each opening mechanism includes a door lock striker, a latch, and a door handle. When the door is closed, the striker engages the latch and holds the door in a closed state. When the door is in a closed state, manipulation of the handle releases the engagement between the striker and the latch. This enables the opening of the door.
The opening mechanism further includes a locking apparatus. The locking apparatus has an actuating lever, and a lock lever, which is connected to the lever by a coupling mechanism. The lock lever is selectively shifted between a lock position and an unlock position. The actuation lever cooperates with the lock lever and shifts between a lock position and an unlock position. The shifting of the lock lever to the lock position arranges the actuation lever at the lock position and causes the locking apparatus to become operative. This prohibits the releasing of the engagement between the striker and the latch. As a result, the engagement between the striker and the latch is not released even when the door handle is manipulated. In other words, the door is held in a locked state and is prohibited from being opened. Shifting the lock lever to the unlock position arranges the actuation lever at the unlock position and causes the locking apparatus to become inoperative. This allows releasing of the engagement between the striker and the latch. As a result, the engagement between the striker and the latch is released when the door handle is manipulated. In other words, the door is held in an unlocked state in which it may be opened.
Each locking mechanism further includes an electric actuator. Each electric actuator selectively shifts the actuation lever of the locking apparatus between a lock position and an unlock position. There is an existing type of electric actuator that is provided with an electric motor, an output shaft, and a gear mechanism connecting to the motor and the output shaft. Actuation of the motor causes linear reciprocation of the output shaft by means of the gear mechanism. When a lock control switch provided near the driver's seat is turned on, the motor is rotated in a forward direction so as to move the output shaft to the front. The frontward movement shifts the actuation lever to the lock position. When the lock switch is turned off, the rotation of the motor is reversed so as to move the output shaft to the rear. The rearward movement shifts the actuation lever to the unlock position. Accordingly, the output shaft of the electric actuator moves reciprocally in accordance with the movement of the lock lever. Reciprocation of the output shaft of the electric actuator shifts the lock lever between the lock position and the unlock position.
Japanese Unexamined Patent Publication No. 7-76963 discloses an example of an electric actuator proposed by the assignee of this application. The electric actuator includes an electric motor and a driven gear, which is driven by the motor. The driven gear is provided with an integral rotary shaft that extends along the axis of the gear. A pair of guide pins project from the outer cylindrical surface of the rotary shaft. A cylindrical output shaft is supported by the rotary shaft. The output shaft moves in the axial direction with respect to the rotary shaft without rotating with respect to the rotary shaft. The output shaft has a guided member that projects from its inner cylindrical surface. The guided member is strip-like and includes a cam surface defined thereon. The guided member slides along each guide pin. The output shaft and the rotary shaft constitute an inverse cam mechanism. Rotation of the rotary shaft slides and moves the guide pins along the cam surface of the guided member. Since it is required for the output shaft to reciprocate along the rotary shaft in a linear manner, the rotary shaft and the output shaft are long.
Abutting surfaces of the guide pins abut against the restricting surface of the guided member when the output shaft is moved to its stroke end position. This restricts the rotation of the rotary shaft and the driven gear. Impact is produced when the rotation of the electric motor is restricted. For each time the rotation of the electric motor is restricted, a relatively large stress is produced at the basal portion of the rotary shaft, or the coupling portion between the rotary shaft and the driven gear. Thus, the rotary shaft must have strength that is sufficient to endure the stress. To satisfy this requirement, it is necessary to either form the rotary shaft from a material having high strength or to enlarge the shaft.
Japanese Unexamined Utility Model Publication Nos. 2-81880 and 2-81881 discloses another type of electric actuator. This electric actuator includes a drive gear (worm gear) provided on the drive shaft of the electric motor, a driven gear (worm wheel), a pinion, and an inclining arm. The driven gear and the pinion are coaxial. The drive gear and driven gear are meshed with each other. The driven gear rotates the pinion. The inclining arm inclines about a pivot pin. A sector gear is provided at the basal portion of the arm. The sector gear and pinion are meshed with each other. Actuation of the electric motor rotates the pinion by means of the drive gear and the driven gear. The rotation of the pinion moves the sector gear and inclines the inclining arm. When this electric actuator is employed in the locking mechanism, the actuation lever of the locking apparatus is selectively shifted between the lock position and the unlock position in accordance with the inclining of the arm.
The above actuator also produces impact when the rotation of the electric motor is restricted as the movement of the inclining arm becomes restricted. Accordingly, stress may be produced between the sector gear and the pinion. Therefore, this electric actuator has a mechanism to absorb the stress produced between the sector gear and the pinion. The absorbing mechanism includes a spring arranged between the driven gear and the pinion. The rotating force of the driven gear is transmitted to the pinion through the spring. Accordingly, when impact is produced as the rotation of the electric motor is restricted, the impact is absorbed by the spring between the driven gear and the pinion. As a result, the stress produced between the sector gear and the pinion is absorbed.
However, the latter electric actuator, which is provided with the absorbing mechanism, differs from the former electric actuator. Accordingly, the former electric actuator type, which reciprocates the output shaft with the rotary shaft provided on the driven gear, does not include a mechanism to absorb the stress produced in the rotary shaft.